ax25: avoid overflows in ax25_setsockopt()

Commit be639ac6 ("NET: AX.25: Check ioctl arguments to avoid overflows
further down the road") rejects very large arguments, but doesn't
completely fix overflows on 64-bit systems.  Consider the AX25_T2 case.

	int opt;
	...
	if (opt < 1 || opt > ULONG_MAX / HZ) {
		res = -EINVAL;
		break;
	}
	ax25->t2 = opt * HZ;

The 32-bit multiplication opt * HZ would overflow before being assigned
to 64-bit ax25->t2.  This patch changes "opt" to unsigned long.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>